The chemical mechanical polish (CMP) is a common practice in the formation of integrated circuits. Typically, CMP is used for the planarization of semiconductor wafers. CMP takes advantage of the synergetic effect of both physical and chemical forces for the polishing of wafers. It is performed by applying a load force to the back of a wafer while the wafer rests on a polishing pad. A polishing pad is placed against the wafer. Both the polishing pad and the wafer are then counter-rotated while a slurry containing both abrasives and reactive chemicals is passed therebetween. CMP is an effective way to achieve global planarization of wafers.
A truly uniform polishing, however, is difficult to achieve due to various factors. For example, slurries are dispensed either from the top or bottom of the polishing pad. This will result in non-uniformity in polishing rate for different locations of the wafer. For example, if slurries are dispensed from the top, the edges of the wafers typically have higher CMP rates than the centers. Conversely, if slurries are dispensed from the bottom, the centers of the wafers typically have higher CMP rates than the edges. To reduce the non-uniformity in polishing rate, pressures applied on different locations of the wafers are adjusted. If the CMP rate in one region of a wafer is low, a higher pressure is applied to this location to compensate the low CMP rate. The increased pressure also serves the purpose of increasing the overall CMP rate, hence the increase in the throughput.
The method of compensating CMP rates through pressures, however, suffers limitations. Since pressures are typically applied through a single membrane, the increased pressure to one region of the wafer inevitably causes the increase in pressure to neighboring regions, and thus the compensation effect is reduced. Furthermore, high pressures applied to wafers may cause unwanted complexity to the wafers. A new CMP system is thus required to increase the CMP rate and/or improve the CMP uniformity without causing unwanted complexity to the wafers.